Construction and operation of safety dumping system in recovery boilers



Dec.. 3, 1968 H. L; MATTHEWS CONSTRUCTION AND OPERATION OF SAFETY. DUMPING SYSTEM IN RECOVERY BOILERS Original Filed July 22, 1965 4 Sheets-Shem 1 muawwie HERBERT L. M/WTl/wj BY W/ 1963 H. L. MATTHEWS 3,413,936

CONSTRUCTION AND OPERATION OF SAFETY DUMPING SYSTEM IN RECOVERY BOILERS Original Filed July 22. 1965 4 uxxn an l' I a I a I E:- yfi 1E 57 mun/roe zq- HE/eBEkr L. MATHEWS 27 W WM,

Dec. 3, 1968 H. 1.. MATTHEWS 3,413,936

CONSTRUCTION AND OPERATION OF SAFETY DUMPING SYSTEM IN RECOVERY BOILERS Original Filed July 22, 1965 4 heets-Sheet s W INK EN 62 as flfkBerL-mnrmaws By W 1968 H. MATTHEWS 3,413,936

CONSTRUCTION AND OPERATION OF SAFETY DUMPING SYSTEM IN RECOVERY BOILERS Original Filed July 22, 1965 v 4 4 HE/zaEkr 4. MATTHEWS United States Patent 3 Claims. Cl. 110-7 ABSTRACT OF THE DISCLOSURE A recovery boiler of the type employed in the kraft sulphate pulp process. The boiler is provided with a movable floor which enables quick dumping of a hot char bed during an emergency shut down.

This application is a continuation of application Ser. No. 474,100 filed July 22, 1965, now abandoned.

This invention relates to an improvement in the manner of construction and the method of operation of a recovery boiler of the type employed for example in the so-called kraft sulphate pulp process in the pulp and paper industry.

More specifically, the invention is directed towards a recovery boiler which constitutes a black liquor recovery unit. In the production of pulp, wood chips are cooked in digestors with a white liquor of which the active chemicals are sodium hydroxide and sodium sulphide. This action separates the lignin from the cellulose of the wood, the lignin being the binder material that holds the cellulose fibres together. The cellulose upon separation from the lignin, provides the pulp for the manufacture of paper.

The residual black liquor containing the spent chemicals and the dissolved lignin is collected, because it is possible to recover the chemicals from this liquor by burning olf the lignin. Such recovery of the chemicals is necessary, in order to make the process economically feasible, and this is the prime purpose of a recovery boiler. However, a great deal of heat is generated in the burning process and it has become conventional'to convert this heat to steam for general use in the plant.

In order to provide a fuller understanding of the typical structure of this type of boiler, which is now extensively used by the industry, reference will now be made to the accompanying drawings, for which purpose a list of such drawings is first set out.

FIGURE 1 shows the main parts of a recovery boiler of the type referred to above, at least those parts relevant to the present considerations, and also shows modifications to the boiler structure in accordance with the present invention;

FIGURE 2 is a fragmentary view of a second embodiment;

FIGURE 3 is a partly cut-away section on the line III-III in FIGURE 2;

FIGURE 4 is a plan view of yet another construction;

FIGURE 5 is a section on VV in FIGURE 4;

FIGURE 6 is a view on VIVI in FIGURE 4;

FIGURE 7 is a plan view of yet another construction;

FIGURE 8 is a side view of a further construction; and

FIGURE 9 demonstrates a manner of dumping the hot char 'bed into a pit.

The conventional parts of the boiler construction illustrated in FIGURE 1 comprises essentially a large, rectangular, vertically elongated enclosure 10 hung from steel beams 11 beneath the roof of the building. The en- 3,413,936 Patented Dec. 3, 1968 closure is formed by means of walls composed of an array of vertically disposed tubes 12 joined together by webs. In operation the tubes contain water, and they are interconnected by headers shown only generally at 13a beneath the floor of the boiler and also communicate with water storage tanks 14 and superheating and screen tubes 15 arranged in the upper part of the enclosure. The tubes 15 which are provided in addition to the tubes 12 that form the bounding walls of the boiler are located generally above the central area where combustion takes place.

A forced draft fan 20 is provided to blow air through ducts 21 and 22 which in turn lead to further ducts that surround the four walls of the boiler at different levels. These ducts serve to supply a draft of air to a series of primary air ports 26, a series of secondary air ports 27 arranged at a higher elevation than ports 26, and a series of tertiary air ports 28 arranged at a still higher elevation in the boiler. In practice, these ports are formed by omitting short lengths of web between the tubes 12 and bending short sections of such tubes to form openings communicating with the interior of the boiler. Apart from an access port which is used when the boiler is shut down and is closed during operation, the air ports 26, 27 and 28 provide the only means of access for air to the interior of the boiler enclosure. An induced draft fan is also provided, and has been illustrated diagrammatically at 30 in FIGURE 1. In practice, the installation is normally more complex than has been shown, including a cyclone evaporator where the flue gases are brought into contact with the black liquor to increase the solids content thereof. These components are in no way modified by the present invention and a detailed description of them would be irrelevant for the purposes of the present specification.

Prior to the boiler being placed in service a fire bed 40 is formed above the floor, this floor being made of chrome dust which is hammered down into hard, brick-like form, the floor together with the lower portions of the walls constituting an imperforate retaining structure for a char bed that will include liquid materials, except for a drain duct 45 described below.

A spray 41 for the black liquor is provided in one of the walls of the boiler. A typical such black liquor spray is arranged to oscillate and to spray the black liquor all around the inside of the boiler, down towards the floor and towards all parts of the wall for the liquor to adhere at least in part to the walls, and more particularly to studs provided on the lower portions of the tubes 12. In addition to the black liquor spray 41, of which there may be more than one, a series of oil burners 42 are arranged around the sides of the boiler. Typically, a few such burners will be provided at a level above the primary ports 26, and some will be located above the secondary ports 27.

When the boiler is started up, black liquor is sprayed onto the fire bed 40 from the spray 41 and oil is sprayed in through the burners 42 and ignited by any suitable means. The burning oil causes combustion of the black liquor and, after operation for a short time, there builds up in the centre of the boiler enclosure a roughly conical heap of burning material which is represented diagrammatically in FIGURE 1 by the broken line 44. This heap of burning material is referred to as the char bed and will burn at a temperature typically somewhere within the range of 1200 to 1500 F., but sometimes in part as high as 1800 F. Once the fire has thus been thoroughly started, the oil burners are turned off and no further oil is normally applied to the boiler. Indeed the burners are normally withdrawn from the walls at this time. Combustion continues from the black liquor which continues to be supplied from the spray 41. The char bed yields a liquor called smelt which contains the chemicals to be recovered, the lignin having provided the source of heat and being largely converted to gas. In addition to combustion of the lignin, the sodium sulphate in the black liquor combines with carbon to produce sodium sulphide, carbon monoxide and carbon dioxide. Sodium carbonate is also formed from the lignin. The smelt is drained off through a duct 45 for further treatment in a conventional manner in a hot water dissolving tank 46.

The forced draft fan and the induced draft fan combine to maintain a continuous flow of air upwardly from the char bed through the boiler, thus maintaining and encouraging combustion at the char bed and removing the products of combustion which thus heat the superheating and screen tubes 15 in the upper part of the boiler, while the char bed itself mainly imparts its heat to the water in the wall tubes 12.

The air supplied through the primary ports 26 is at a comparatively low pressure in order to promote a reducing atmosphere in the burning mass of char. The secondary ports 27 which are about 5 to 6 feet above the primary ports 26 have air supplied through them at a slightly higher pressure, in order to promote burning of combustible materials rising from the glowing mass of the char bed, while the air supplied to the tertiary ports 28 is at a still higher pressure, being designed to further and complete the combustion process.

As has been mentioned, the oil burners 42 are not permanently fixed in the unit. They are inserted through ports at approximately the level of the secondary ports 27, as and when they are needed. Normally these burners 42 are only required for starting up the boiler, for operation under adverse operating conditions, and for burning down the char bed when shutting down the boiler. For shut down, the supply of black liquor is turned off and sufiicient oil is supplied to ensure that the char bed is fully consumed. Typically, such a shut down operation takes from 4 to 6 hours.

When the boiler is operating normally, the char bed 44 assumes the shape of an irregular heap the sides of which are approximately on a level with the primary ports 26, while its high points are approximately on a level with the secondary ports 27. Drying combustible material is continuously falling on top of this heap, as the organic material is burnt oflf. Such combustible material comes directly from the spray 41 and also falls from the side and back walls on which it has become partially dried and caked as it runs down the faces of the wall tubes 12.

All the parts of the structure and the method of operation that have so far' been described relate to conventional boiler construction and operation.

One of the principal difficulties in operating this type of boiler is the danger of explosion. In recent years several men have lost their lives or been seriously injured in pulp mill explosions, and extensive damage to plant has been incurred. The danger of explosion arises from leakage of water from the tubes, some measure of which is virtually inevitable at some time or other. There are so many tubes, all subjected to rigorous temperature conditions, that, as a practical matter, it is impossible to operate for any length of time without tolerating a certain amount of water leakage. Serious leaks require immediate shut down of the boiler, but naturally, it is desirable to keep the number of shut downs to a minimum. The operating time lost by a shut down is substantial and this is particularly true in the case of an emergency shut down when no time had been available for first consuming the char bed. If the boiler is allowed to cool with the char bed unconsumed, its subsequent removal when cold is a lengthy, expensive and at times dangerous undertaking.

The novel concept of the present invention is to provide the boiler with a movable floor structure whereby the hot char bed can be quickly dumped from the boiler and conveyed to a relatively safe location. An important factor is that the char bed should be removable from the boiler as quickly as possible. Once a serious leak has developed time is often of the essence if the boiler is to be saved. Some boilers have exploded in a matter of minutes. There may not be time to burn down the char bed.

Various methods are possible for carrying the present inventive concept into practice and some of these are illustrated by way of example in the accompanying drawmgs.

As FIGURE 1 demonstrates the tube structure of the boiler is modified by dispensing with the conventional floor tubes and bringing the wall tubes 12 down to the headers 13a which extend around the four sides of the boiler without extending across the space directly under a floor on which the fire bed 40 is constructed and the char bed 44 is formed during operation. In the FIGURE 1 construction the floor 50 is supported around its four edges by a series of removable pins 51 of which two representative ones are shown. It will be evident that, if the pins 51 are simultaneously withdrawn by electrical or mechanical operating mechanisms 52, the floor 50 and the char bed thereon will be free to fall down into a suitable pit (not shown) where the danger of explosion can be minimized, because such pit can be made free of the leaking water that is the basic cause of the danger.

An'arrangement in which the boiler floor opens up to discharge the char bed, and then recloses is in many respects preferred and such an arrangement is shown in FIGURES 2 and 3. Here the floor is formed as a pair of doors 53 hinged at 54 and held up in their normal operating posi ion by hydraulic cylinder assemblies 55. Release of pressure in the assemblies 55 will allow the doors to fall open to the vertical emergency position shown in broken lines. For added certainty of operation, the hydraulic assemblies can be made double acting, so that the doors can be positively drawn down to the open position. After the char bed has been dumped in this way, the hydraulic assemblies 55 can be reactivated to reclose the doors 53 and thus positively shut off the boiler interior from the pit into which the char bed has been dumped. If desired, safety pins 56 may be employed during normal operation to hold the doors 53 closed, pins 56 being withdrawn by mechanisms 57 before dumping.

FIGURES 4 to 6 illustrate a modification of this construction in which four doors 60 are employed, each hinged at 61, and interconnected for rotation in unison by a drive mechanism 62 operated by a motor 63. The principle of operation is basically the same as before, and again pins 64 may be used during normal operation for safety.

FIGURE 7 shows diagrammatically a modification whereby the floor is divided diagonally into four triangular doors 65 rotatable about interconnected hinges 66 by a motor and gear assembly 67.

FIGURE 8 shows yet another construction in which the fioor 68 is retained in one piece, while being capable of being lowered differentially, as indicated in broken lines by hydraulic rams 69 and 70, whereby to discharge the char bed 44 into a pit 71.

FIGURE 9 illustrates a possible arrangement of a pit 72 for use in association with the other embodiments of the invention, the hot char bed material being discharged down chute 73 from the boiler 10 which is supported from the top in the usual way. By this means the pit 72 can be arranged outside the building 74 in which boiler 10 is housed. Alternatively, and if preferred, the pit can be disposed directly beneath the boiler, suitable access from the side being provided for disposing of the char bed once it has cooled down. In any event, provision will be made, either by ventilation or by locating the pit in the open atmosphere, to ensure that no explosive mixture of gases can accumulate.

In the constructions illustrated the floors have been shown as solid metallic construction, and they may of course incorporate insulating material. If desired, they may be water cooled, where appropriate, such as in the hinged constructions, the doors then being formed with a series of tubes and the water being led in and out along hollow hinge constructions.

With each of the constructions illustrated, the emergency shut down procedure will consist of shutting off the black liquor spray 41, shutting off the forced draft fan 20, shutting ofi or reversing the induction fan 30, and opening the floor structure to dump the char bed. The building may also be evacuated of personnel, depending on the severity of the emergency. The floor structure may be returned to its original position to minimize any subsequent fiow of leaking water out of the boiler, such action being taken at the discretion of the engineer in charge in the light of all the circumstances.

The provision of a movable floor structure, and more particularly, a floor formed as hinged doors, has further advantages over and above the prime advantage of safety. It reduces normal shut down time, because the doors may be used to dump a hot char bed under routine as well as under emergency shut down conditions. It also facilitates repairs and cleaning of the boiler, because of the greater access to the boiler interior that it provides.

I claim:

1. In a recovery boiler of the type employed in the sulphate pulp process to recover chemicals from black liquid and to generate steam, the said boiler having wall means defining a vertically extending enclosure and a floor structure on which a viscous char bed of burning black liquor is formed, said floor structure forming a substantially imperforate planar surface extending in :an inclined plane throughout the lower cross section of said enclosure, a drain duct disposed at the lower portion of said enclosure coplanar with the upper surface of the lower portion of said inclined floor structure; the improvement comprising adjustable means mounting said fioor structure to be movable between a normal operating condition in which said inclined surface supports said char bed and said floor structure is substantially imperforate throughout the lower cross section of said enclosure and an emergency dumping condition in which said floor structure is in an open and lowered condition to discharge said char bed downwardly to a location remote from said boiler enclosure and at least a portion of said planar surface being inclined :away from said drain duct when said mounting means is actuated to move said floor structure to the open condition.

2. A recovery boiler according to claim 1, wherein said floor structure comprises a plurality of hinged doors, each said door movable between said operating condition and said emergency condition and at least one of said doors inclined away from said drain duct when said floor structure is in said emergency condition.

3. A recovery boiler according to claim 1, including releasable safety means for positively holding said floor structure in said. operating condition during normal operation of the boiler.

References Cited UNITED STATES PATENTS 2,498,787 2/1950 Bowsman 165 X 2,933,057 4/1960 Wilson l107 3,168,074 2/1965 Kuhner et a1 1107 X 3,215,099 11/1965 Ooulter 110-7 3,225,721 12/1965 Rowley 110-7 JAMES W. WESTHAVER, Primary Examiner. 

